112 



growing, the rectrices are also growing with distinct hornsheaths. 



Bill 30, height of bill 8.5, wing about 130, tarsus about 32 ram. 

 All measures taken from the dry skin. 

 Sex ?, 15 October 1921. Brown juvenile-feathers on the back, in smaller 

 number than in the first bird. Remiges growing, rectrices seem to 

 be f'ullgrown. Some of the new shoulderfeathers are narrowly edged 

 with white at the tip. Primary-coverts don't seem to be moulted. 



Bill 28, height of bill 9, wing about 160, tarsus about 32 mm. 

 All measures taken from the dry skin. 



Maturity: As said before, Guillemots and Razorbills, as a rule, 

 do not breed in their first summer. The large numbers of idlers 

 which are to be found in the vicinity of the breeding-colonies in 

 summer and also the birds which are met with in summer on sea 

 far from land or along our flat coasts, without much doubt are 

 composed for the greater part of these immature birds. Occasionally 

 a one year old bird seems to come to coupling, but if further 

 examination might confirm this opinion, the case will most likely 

 be just as rare as that of some birds which don't come to breeding 

 in their second summer. The reason why the latter birds don't breed, 

 may be sterelity; the breeding of the former ones might be expected 

 from very strong and early hatched specimens (see however post- 

 scriptum). 



Sterile birds may be less rare than we may be inclined to believe. 

 There namely occur Razorbills with three furrows in the bill, whereas 

 their genitals have not been in function the preceding summer or before. 

 These birds must be either: 



a. birds in their second winter, which received three billfurrows during 

 the first moult into breedingplumage or 



b. birds in their third or later winter, which for some reason did not 

 use their genitals. 



As this question seems to me of much interest, I give the corresponding 

 observations, all concerning Razorbills, below : 



9 — 3 furrows — 19 November 1921 — Oviduct not having been 



used ? 



(j^ — 3 furrows — 12 January 1922 — Testicles have not been used. 



9 — 3 furrows — 12 January 1922 — Oviduct has not been used. 



9 — 3 furrows and the 4th one indicated indistinctly — 10 March 



1922 — „Strongly developed ovary! but oviduct in my opinion 



doubtless not having been used". 



I, for myself, am sure that these birds all belong to b, that they are 



birds in their third or even in a later winter and — though I can't 



prove this — woi birds in their second one. Especially as the last named 



